Very low HOH bending vibrations. V. Quantum chemical study of water bending vibrations in MgKPO4·H2O

The geometry and harmonic vibrational frequencies of a water molecule placed in fragments of the crystal surrounding in MgKPO4·H2O were studied by the finite-cluster methodology at the Hartree–Fock (HF) and B3LYP/6-31þþG(d,p) levels of theory. Several finite clusters representing the surrounding of the water molecules in the structure of MgKPO4·H2O were considered: Mg2þKþ(H2O), Mg2þKþ(PO4 32)2 (H2O), Mg2þKþ(PO4 32)4(H2O) and the corresponding charge–field perturbation theoretical analogues. The results for the smallest clusters indicate that the purely electrostatic interaction with the Mg2þ and Kþ cations leads to a relatively small dðHOHÞ frequency upshift, significantly smaller than the full-wavefunction value, which is approximately þ70 cm21 regardless of the level of theory. The addition of two nearest PO4 32 anions leads to a significant ‘flattening’ of the dðHOHÞ vibrational potential, accompanied by an enormous frequency downshift of more than 350 cm21. However, the HF results for the largest of the considered clusters (including four nearest PO4 32 anions besides the Mg2þ and Kþ cations) are in a good agreement with the experimental data, whereas the B3LYP level of theory gives somewhat smaller overall frequency downshift, although the predicted trend is correct. The validity of the obtained results and the possible directions for further improvement of the applied methodology are discussed. q 2004 Elsevier B.V. All rights reserved.